1. Field of the Invention
The present invention relates to a semiconductor device and a manufacturing method thereof, and more particularly to a semiconductor device which is suitable for miniaturization and a manufacturing method thereof.
2. Description of the Related Art
As miniaturization of semiconductor devices progresses, various technologies have been developed in order to reduce an area of a semiconductor device.
An example of a prior art will now be described with reference to a non-volatile semiconductor storage device disclosed in U.S. Pat. No. 6,777,741. In the technology of this patent, a semiconductor film which serves as a part of a floating gate electrode is formed on a flat silicon substrate via a tunnel insulator. A groove for isolation is formed in the semiconductor film, the tunnel insulator and the silicon substrate, and then an isolation is formed in the groove. Furthermore, a second semiconductor film is selectively formed on the semiconductor film, whereby a floating gate electrode having a two-layered configuration is formed. In this technology, a width of the isolation is hard to be set narrower than a resolution of a lithography used.
In another technology, an isolation protruding from a semiconductor substrate is formed in advance. On the semiconductor substrate between the isolations, a semiconductor device, e.g., a metal oxide semiconductor field effect transistor (MOSFET), is formed. According to this technology, an alignment accuracy between the isolation formed in the semiconductor substrate and an active element formed on the semiconductor substrate can be improved, thereby a packing density of the semiconductor device can be improved. However, this technology has following problems.
FIG. 14 shows an example of a cross-sectional structure of a channel portion of a memory cell of a non-volatile semiconductor storage device using this technology. The illustrated structure is formed as follows.
First, a mask material (not shown) is deposited on a silicon substrate 10, an isolation groove 16t is formed in the mask material and the silicon substrate 10 by lithography and etching. The isolation groove 16t is filled with an insulator, thereby forming the isolation 16. After forming the isolations 16, the mask material is removed to expose a surface of the silicon substrate 10 between the isolations 16, and cell transistors 20 of the memory cell are formed on the exposed silicon surface. During removal of the mask material, a part of the isolation 16 adjacent to the silicon substrate is also removed, and thus an upper portion of the silicon substrate 10 is configured to slightly protrude as shown in FIG. 14. A floating gate electrode 24 is formed on this protruding silicon substrate 10 via a tunnel insulator 22.
A film thickness of the tunnel insulator 22 formed on the non-flat silicon substrate 10 varies depending on the shape of the substrate, which is flat on an upper surface and inclined on an end portion of the silicon substrate 10. In other word, the tunnel insulator 22 on the inclined part of the silicon substrate 10 is thinner than that on the flat part. As a result, a parasitic transistor 20′ is formed on the inclined area, and the parasitic transistor 20′ has characteristics different from those of the cell transistor 20 formed on the flat portion. When the parasitic transistor 20′ is formed, variations are generated in characteristics of the semiconductor device, such as a breakdown voltage and tunneling current of the tunnel insulator. Variation in the tunneling current among the memory cells causes nonuniformity in an amount of electric charge injected into each floating gate electrode 24. Further, the floating gate electrode 24 has a shape protruding downwards on both sides, thereby electric field concentration is apt to occur there. These phenomena are combined so that electrical characteristics of the cell transistor 20 are deteriorated. For example, a kink is produced in gate voltage-source/drain current characteristics (I-V characteristics) or variation in a threshold voltage Vth is generated.
Furthermore, since a width of a channel area becomes narrower in a miniaturized MOSFET, there is another problem that source/drain current is difficult to increase.
Therefore, there is a need for a semiconductor device suitable for miniaturization and a manufacturing method thereof which can improve variations in characteristics of a transistor mentioned above and enhance a current driving capability.